Plethon’s Critique of Aristotle’s Theory of Sense Perception in the Light of the 15th-Century Controversy on the Philosophy of Plato and Aristotle

In Categories 7b36–38 Aristotle prioritized the object of sense perception over the act of perception itself, observing that the withdrawal of the perceptible (αἰσθητόν) entails the cancelation of perception (αἴσθησις), while the removal of the act of perception leaves the perceptible subsisting. This last point was enough for Plethon to initiate his own critique, advocating that Aristotle did not seem to have endeavoured a solid coalescence between the problems raised in his theory of knowledge and the issues elaborated in his Metaphysics. In an attempt to present these two fields of inquiry as in greater harmony with each other and to shed light on what he considered to be the weak points and contradictions of Aristotle’s theory of knowledge, Plethon claimed that Aristotle’s view seemed to disprove his own vision of καθόλου λόγος, especially insofar as the category of relation (πρός τι) is involved. Plethon conceived of relation as of the simultaneous and necessary character of the natural concurrence (ἅμα τῇ φύσει, Cat., 14 b 27–28) between the act of perception and its object. Thus, for him, Aristotle’s approach should be thought of as false or, at least, as inadequate; first, because an object and an act of perception must always concur naturally and, second, because, in accordance with Metaphysics 1010 b 30–32 and De anima 425 b 25, if sensible perception (αἴσθησις) is not sustained or is withdrawn, the object (αἰσθητόν) of a non-subsisting sense perception cannot subsist. On the other hand, if sense perception is to subsist in the future, it is obvious that it will appear out of something potentially subsisting (δυνάμει ὑπάρχον). Thus, for Plethon, the object of sense perception is both the potentially perceptible (δυνάμει αἰσθητόν) and perception in relation to its potency (πρὸς δυνάμει αἴσθησις).